Known piezo-electric motors include on the one hand, a stator on which are mounted piezo-electric means, such as a polarised ceramic provided with excitation electrodes adapted to be electrically energised, and on the other hand, a rotor which rests axially on the stator and which is assembled for rotation relative to the latter around a driving shaft mechanically coupled to a mechanism to be driven.
Under the effect of the electrical excitation received from the electrodes, the ceramic transmits a vibratory motion to the stator which induces the rotational displacement of the rotor arranged to bear elastically thereon.
In this type of asynchronous motor, the angular position of the rotor is, after rotation, intrinsically unknown, which is a major inconvenience if the rotor is to drive the hands or the date-disc of a timepiece.
In patent application JP-A-60 113 765, a piezo-electric motor for a timepiece is disclosed comprising an angular position detector of the rotor. The motor comprises in a known manner a stator on which are mounted piezo-electric elements provided with electrical excitation electrodes, and a rotor assembled rotationally relative to the stator and provided with one or several holes. Above the holes formed in the rotor is assembled a Light Emitting Diode (LED) which is secured to a separate substrate which is fixedly attached to the motor. A photo-transistor is placed on a substrate positioned under the rotor, at right angles and in line with the LED. When the light emitted by the LED passes through the holes of the rotor, the photo-transistor will receive this emitted light and thus the angular position of the rotor can then be detected.
The size of the motor provided with this detector is large. Furthermore, the motor comprises a large number of parts which increase its price and decrease its reliability.
Another solution for detecting the position of the rotor is known from document JP-A-60 51 478 which proposes means for detecting the position of the rotor comprising an electrical contact system formed by a fixed brush, which rubs over a mobile disc on which 60 conducting contact-points are formed (collector). This detection disc, which is under tension, is fixed to the rotor. This arrangement also presents the inconvenience of having a large number of components. Moreover, the assembly of the motor is rather complicated because the detection disc must be centred onto the rotor and put under tension, and this for every single motor. Also, the contact between the brush and the contact-points causes a friction opposed to the rotary movement of the rotor so that the brush tends to wear out rapidly.
Furthermore, the wear of the contact-surface is difficult to master and the presence of dirt-particles caused by this wear can prevent normal functioning of the motor.
The aim of the present invention is to find a remedy for these inconveniences by providing a piezo-electric motor comprising a position detector being of very small dimension, which may be easily manufactured and which is of low cost.